1. Field of the Invention
The present invention relates to ion-exchange materials and, more particularly, this invention relates to novel ion-exchange hollow fibers and methods of forming and using such fibers.
2. History of the Prior Art
Current technology for removal of ions from dilute streams is largely oriented to the use of conventional packed, ion-exchange beds. These processes, however, have their problems. There is, for example, significant current effort toward the development of macroreticular pores in the ion-exchange beads which would be less susceptible to irreversible clogging. There are problems in the preparation of beads which have adequate porosity but which are still not unduly fragile. In the preparation of commerical ion-exchange beads, the process is as follows:
A cross-linked polymer bead is formed by reacting, for example, styrene and divinylbenzene. The percentage of cross-linker (divinylbenzene) determines the extent of swelling in the final bead as ions are exchanged. The greater the percentage of cross-linker, the less the swelling. Concurrently, the greater the level of cross-linker, the slower will be the diffusion of exchanging ions into and out of the beads, and the slower will be the process.
After the bead is formed, a chemical reaction such as sulfonation or chlormethylation is used to form the ion-exchange sites. From the description it is apparent that there are conflicting demands: high cross-link density helps stability but reduces product rate. Similarly, high ion-exchange capacity from the second step induces large swelling excursions, but provides greater capacity. Swelling of the resin beads occurs due to the osmotic pressures which are generated when the beads are exposed to different concentrations of various electrolytes. Pressure drop build-up is irregular and troublesome in regeneration processes. The choice of operating cycles is not straightforward at all and the beads are not inexpensive.
An alternative exists in semipermeable flat membranes but the technology is still in its infancy and the costs to efficiency ratio of membrane processes is not very satisfactory. Ion-exchange membranes offer significant adavantages in separation processes with respect to ion-exchange resin beads. When the ion-exchange resins are in the form of membranes, they can be in contact with the solution to be separated and the stripping solution simultaneously and the ion-exchange process can be continuous rather than cyclic.
A continuous process with ion-exchange membranes has several advantages over an ion-exchange resin column. Some of these are (1) a separate regeneration step is eliminated, (2) the product has a constant composition, (3) the hold-up volume is low, and (4) operation and control of the process are simple. The controlling factors with respect to rate of transport of ions across such membranes are as follows: (a) The membrane must be thin so that the resistance to diffusion is minimal. (b) The membrane must be semipermeable, i.e., if the membrane has positive charges they must be so uniformly and closely distributed that any solid trying to permeate the structure will always see some of the fixed charges. The consequence of this distribution is that only ions opposite in charge to the fixed charges can permeate the membrane. All ions having the same charges as the fixed charge are excluded by electrostatic repulsion. (c) The resistance to hydraulic permeability should be as low as possible consistent with the above requirements. Since this requirement and the preceding requirement are conflicting, all practical membranes are a compromise of these two objectives. (d) The flux of ions across all membranes is proportional to the area available for transfer.
These requirements result in certain limitations of the flat membrane system. The productivity per unit volume is unsatisfactory and the membranes must be supported in any separation device. Ion-exchange membranes cannot be manufactured by the same techniques utilized to form ion-exchange beads since the swelling resulting from the formation of the ion-exchange site is too great to be borne by membranes which have a low degree of cross-linking. However, if the degree of cross-linking is raised, the membrane is too brittle to be useful. Most flat ion-exchange membranes are formed by first forming ion-exchange beads and then milling the beads into a thermoplastic resin as a binder for the resin structure. In a more recent process, the thermoplastic resin is milled in the presence of a swelling agent which is then replaced with a graftable ionic monomer. After grafting, the ionic site is bound to the membrane. The mechanical requirements are satisfied by using relatively thick sheets, in the range of 100-300 microns.
The hollow fiber configuration of a membrane offers the opportunity to prepare thin-walled devices with very large surface areas. Such a device would also provide the flexibility of high transport rates per unit volume and the possibility of continuous operation without the need for regeneration cycles. Further advantage over other configurations is that supports are not required for the hollow fibers.
Anionic exchange hollow fibers have not been reported. Sulfonic acid cationic exchange type of hollow fibers have been prepared by irradiating polyethylene hollow fibers, immersing the irradiated fibers in styrene and heating the mixture to effect grafting. The fibers are then swollen in dichloromethane and suflonated with chlorsulfonic acid, followed by hydrolysis. This procedure requires several steps, effects a random ion-exchange capacity and is limited to special reactants. Post-treatment of hollow fibers is further limited since the very small cross-section of the fibers and the fine porosity of the walls prevents introduction of preformed polymers into the bore or impregnation into the walls.